But, for now at least, there is a significant catch. The egg-free ESCs are abnormal, containing an extra set of chromosomes, and so cannot be used for treating patients.

Nonetheless, Kevin Eggan and his colleagues who pioneered the breakthrough at Harvard University in Cambridge, Massachusetts, US, say the new technique is ideal for enabling scientific research to find how to turn adult cells directly into ESCs.

At present, patient-matched ESCs can only be created through the same cloning technique which produced Dolly the sheep – a process to which some people object because in humans it would involve destruction of human eggs and embryos. Human eggs are also difficult to obtain and to work with.

When fused with an adult cell, eggs emptied of their own nuclear DNA reprogram the adult cells to form an embryo, from which ESCs can be harvested. These cells can differentiate to form all tissues in the body. Theoretically, they could be created from a patient's own adult cells - from skin, for example - and used to repair tissue damage without the complications of tissue rejection by the immune system.

Now, Eggan and his colleagues have shown that pre-existing ESCs originally extracted from spare IVF embryos are just as good as eggs at reprogramming adult cells. And the pre-existing ESCs could come from a self-renewing cell line and be essentially inexhaustible.

Complete solution

When the researchers fused pre-existing ESCs with foreskin cells, components of the ESCs reprogrammed the foreskin cells so that they became new ESCs. But the new ESCs still contain chromosomes from the original ESCs which did the reprogramming, as well as the chromosomes from the foreskin cells.

“If we can figure out how to remove the ESC chromosomes before or after fusion, it could be a complete solution and you’d no longer need eggs,” says Eggan. “The cells we’ve made now will never be useful for transplant or understanding disease, but the process will give us an alternative to eggs as a source of material.”

Eggan also acknowledges that because the ESCs used for reprogramming originally came from spare IVF embryos, they “carry the same moral burden” as eggs. But US president George W Bush solved a related political dilemma over stem cells in 2001 by allowing work to continue on existing stem cells lines derived from embryos, while banning the federal funding of new lines.

Problem solved?

Other researchers already claim to have solved the problem of the extra chromosomes. Yuri Verlinsky and his colleagues at the Reproductive Genetics Institute in Chicago, US, claimed in May to have achieved the same as Eggan, but by reprogramming with ESCs stripped beforehand of their nuclear DNA. Without this extra genetic baggage, the cells have real potential for therapy.

Verlinsky has yet to reveal how he does this, but says that the secret will appear soon in a paper submitted for full publication. “For us, it’s the easiest part,” he told New Scientist.

Verlinsky welcomed the Eggan breakthrough, published in Science. “It validates what we did,” he says.

Eggan expressed great interest in independent attempts by Verlinsky and by Alan Trounson of Monash University in Australia to exclude the ESC chromosomes. “They claim to have some success, but I’ve not seen any of the data and it’s not been published,” he says.